1. Field of the Invention
The present invention relates to a fabricating method of a semiconductor device for use with various electronic units.
2. Description of the Related Art
According to the demand of a smaller and thinner semiconductor device, a semiconductor element (chip) has been connected by use of flip-chip bonding. In the flip-chip bonding method, a semiconductor chip is mounted with a face down (with an electrode pad formed surface downward) on a wiring substrate. Protruded electrode composed of for example solder (hereinafter referred to as bumps) formed on an electrode pad of the semiconductor chip is pressed to a connecting pad of the wiring substrate and then heated. Thus, the solder is reflowed and thereby the electrode pad is connected to the connecting pad of the wiring substrate. Alternatively, solder bump may be formed on a connecting pad of a wiring substrate. The flip-chip bonding method is superior to the wire bonding method in the mount density.
In a flip-chip bonded semiconductor device, the thermal expansion coefficient of a semiconductor chip composed of silicon or the like is largely different from that of a wiring substrate (for example, a glass cloth-epoxy resin impregnated wiring substrate). The stress due to the difference of the thermal expansion coefficients between the semiconductor chip and the wiring substrate is applied to solder bumps that connect the semiconductor chip and the wiring substrate, the connected portion being deteriorated.
To prevent such a problem, after solder is reflowed, as shown in FIG. 8, liquid resin 22 such as epoxy resin is filled in a space (gap portion) between a semiconductor chip 20 an a wiring substrate 21 due to capillary phenomenon. Thus, a resin encapsulation layer (referred to as underfill) is formed. The resin encapsulation layer alleviates the thermal stress due to the difference of the thermal expansion coefficients between the wiring substrate and the semiconductor chip. In addition, the resin encapsulation layer reinforces and mechanically protects the flip-chip bonded portion. In FIG. 8, reference numeral 23 denotes solder bumps. Reference numeral 24 denotes a dispenser that dispenses drops of liquid resin.
However, in such a method of forming a resin encapsulation layer, it takes a long time to fill liquid resin. In addition, the dispersibility of a filler contained in the liquid resin deteriorates on the flow end side thereof. Thus, a flow mark will take place.
To fabricate a flip-chip bonded semiconductor device, a simultaneous connecting method is known. In the simultaneous connecting method, while-bumps are being heated and connected (solder is reflowed), the resin encapsulation layer is formed.
In the method, as shown in FIGS. 9 and 10, after fluid resin 25 that contains flux or a reductive material is coated on a wiring substrate 21 (FIG. 10A), at a flip-chip bonding step, the semiconductor chip 20 is mounted. Bumps 23 are heated and melted (reflowed). In addition, the height of the bumps 23 is controlled (FIG. 10B). Thereafter, the fluid resin 25 layer is heated and hardened. The height of the bumps 23 is controlled so as to keep the space between the semiconductor chip 20 and the wiring substrate 21 with a predetermined size.
Thereafter, when necessary, as shown in FIG. 10C, a cover plate 26 is placed on the upper surface (the opposite surface of the electrode pad formed surface) of the semiconductor chip 20. The cover plate 26 is adhered to the semiconductor chip 20 with adhesive agent 27.
However, in the simultaneous connecting method, since the fluid resin 25 is coated on the connecting pad of the wiring substrate 21, the fluid resin 25 resides at the connection interface of the bumps 23. Thus, a connection defect tends to take place. To prevent the connection defect due to adhesion and residue of such resin, an extra step is required along with the flip-chip bonding step. However, it is difficult to adjust each step. Thus, the steps become complicated.
In other words, when the semiconductor chip 20 is mounted, to force aside the fluid resin 25 coated on the bump 23a on the substrate side with the bump 23b on the chip side, the semiconductor chip 20 should be pressed downward. In addition, to prevent the fluid resin 25 from entering the contacted surface of the bumps 23a and 23b, while the semiconductor chip 20 is being pressed, the solder should be heated and melted.
As described above, to alleviate the stress due to the difference of the thermal expansion coefficients between the semiconductor chip 20 and the wiring substrate 21, it is necessary to keep the bumps 23 disposed therebetween with a predetermined height. Thus, while solder is being reflowed, the height of the bumps 23 should be controlled. Consequently, it is difficult to adjust such steps.
In addition, when there are many bumps 23, it is difficult to completely force aside the resin 25 at the connection interface of the bumps 23a on the substrate side and the bumps 23b on the chip side. Thus, a connection defect tends to take place due to the adhesion or residual of the resin 25 at the bumps connection interface.
Thus, in the simultaneous connecting method of which the flip-chip bonding step and resin filling step are simultaneously performed, it is difficult to accomplish a semiconductor device with high connection reliability that satisfies the requirements of small size and many pins.
In addition, when solder bumps are melted, they become round and thereby the connection height becomes lower. Thus, it is difficult to maintain the reliability of the semiconductor device. Further, since the bumps become round, it is difficult to decrease the pitch of the bumps.
The inventors of the present invention know that a fabricating method of a semiconductor device that prevents resin from adhering and residing between an electrode pad and bumps. In the method, a fluid thermosetting adhesive layer is formed on an electrode pad formed surface of a semiconductor chip or on a connecting pad of a wiring substrate. And the adhesive layer is forced aside by bumps.
However, in such a method, when the number of bumps becomes large, it is difficult to completely force aside the adhesive layer that resides at the bump interface with the bumps. Thus, since the adhesive agent layer resides at the bump interface, electric connections become insufficient. In other words, it is difficult to satisfy the requirements of smaller size and more pins of a semiconductor chip.
The present invention is made from the above-described point of view. An object of the present invention is to provide a method of fabricating a semiconductor device with high connection reliability of a flip-chip bonded portion with simplified steps and high yield.
A first aspect of the present invention is a fabricating method of a semiconductor device, comprising the steps of (a) forming a resin layer on a connecting terminal formed surface of a wiring substrate, the wiring substrate having a connecting terminal and a wiring layer on at least one main surface thereof, (b) adhering an inner surface of a cover member on the opposite surface of an electrode terminal formed surface of a semiconductor element, the semiconductor element having an electrode terminal on a main surface thereof, (c) mounting the semiconductor element with the cover member placed with a face down on a surface of which the resin layer of the wiring substrate is formed, contacting a peripheral portion of the inner surface of the cover member, and contacting the electrode terminal of the semiconductor element and the connecting terminal of the wiring substrate through bumps formed on at least one of the electrode terminal and the connecting terminal, and (d) heating and melting the bumps to connect the electrode terminal of the semiconductor element and the connecting terminal of the wiring substrate therewith.
As the bumps, bumps composed of low-melting metals such as solders can be used. In particular, it is preferable to use Pbxe2x80x94Sn solder bumps, Snxe2x80x94Ag solder bumps, or Snxe2x80x94Agxe2x80x94Cu solder bumps.
In the fabricating method of a semiconductor device according to the first aspect, at the bump connecting step, while bumps are being heated, melted, and connected, the resin layer formed on the wiring substrate can be cross-linked and hardened. When the bumps are connected and the resin layer is hardened at the same step, the number of steps can be reduced.
As an example of the cover member, a cap type cover member having a concaved portion in a semiconductor element attached portion can be used. The depth of the concaved portion is set to equal to or smaller than the sum of the height of the bumps being connected (hereinafter simply referred to as the height of bumps) and the thickness of the semiconductor element. The inner surface of the concaved portion is adhered with the opposite surface of the electrode terminal formed surface of the semiconductor element.
In such a structure, in the state that the semiconductor element is mounted on the wiring substrate, since the peripheral portion of the inner surface of the cover member directly contacts the wiring substrate, the bumps connection interface is stably pressed. Since the size of the space (gap portion) between the semiconductor element and the wiring substrate (namely, the height of the bumps) is controlled, it is not necessary to use an extra height controlling means.
As an example of the cover member, a plate member that does not have a concaved portion can be used. In this case, an adhesive layer with the thickness that is equal to or smaller than the sum of the height of the bumps and the thickness of the semiconductor element is formed at the peripheral portion of the inner surface of the cover member. At the semiconductor element mounting step, it is preferable to contact and adhere the adhesive layer and the wiring substrate.
Instead of disposing the adhesive layer on the cover member, a resin layer with the thickness that is equal to or smaller than the sum of the height of the bumps and the thickness of the semiconductor element may be formed in a peripheral area of a semiconductor element mounted portion of the wiring substrate. In the semiconductor element mounting step, the cover member may be contacted to the wiring substrate through the resin layer. As an example of the resin, resist such as solder resist is used. In such a method, the resin layer functions as a dam for an underfill resin layer formed on the connecting terminal formed surface of the wiring substrate. Thus, the resin layer is prevented from excessively spreading out. Consequently, an underfill having a fillet-shaped portion is formed.
According to the first aspect of the present invention, at the step of which a semiconductor element with a cover member is mounted to a wiring substrate, a peripheral portion of a inner surface of a cover member is contacted to the wiring substrate directly or through another layer (an adhesive layer or a resist layer). Thus, the connection interface of bumps between the semiconductor element and the wiring substrate is stably pressed. In such a stably pressing state, the bumps are heated and melted. Thus, the resin for underfill does not adhere, reside, or interpose at the connection interface of the bumps. Consequently, a connection defect will not take place. Thus, the yield will improve.
When a cover member is placed on a semiconductor element, at the bump connecting step, the size of the space between the semiconductor element and the wiring substrate is kept constant. Thus, the height of the bumps is controlled. Consequently, it is not necessary to use a high controlling means for preventing the semiconductor element from lowering. Thus, the process can be simplified. In addition, the number of fabrication steps can be decreased. Moreover, the yield will improve and high throughput can be accomplished.
A second aspect of the present invention is a fabricating method of a semiconductor device, comprising the steps of (a) forming a non-fluid resin layer with a predetermined thickness on a connecting terminal formed surface of a wiring substrate, the wiring substrate having a connecting terminal and a wiring layer on at least one of main surfaces thereof, the non-fluid resin layer being not formed on the connecting terminal, (b) mounting a semiconductor element with a face down on a surface of which the non-fluid resin layer of the wiring substrate is formed, the semiconductor element having an electrode terminal on a main surface thereof, and contacting the electrode terminal of the semiconductor element and the connecting terminal of the wiring substrate through bumps formed on at least one of the electrode terminal and the connecting terminal, and (c) heating, melting the bumps to connect the electrode terminal of the semiconductor element and the connecting terminal of the wiring substrate therewith, and contacting the semiconductor element to the non-fluid resin layer.
As the bumps, bumps composed of low-melting metals such as solders can be used. In particular, it is preferable to use Pbxe2x80x94Sn solder bumps, Snxe2x80x94Ag solder bumps, or Snxe2x80x94Agxe2x80x94Cu solder bumps.
In the fabricating method of a semiconductor device according to the second aspect, at the bump connecting step, while bumps are being heated, melted, and connected, the non-fluid resin layer formed on the wiring substrate can be hardened (for instance, cross-linked). When the bumps are connected and the non-fluid resin layer is hardened at the same step, the number of steps can be reduced.
As an example of the non-fluid resin, thermosetting resin or thermoplastic resin that contain 70 to 90% by weight of a filler such as silica can be used. The non-fluid resin layer can be formed as follows. The resin is formed in a sheet shape or a film shape. A resin sheet or film that has a through-hole corresponding to the connecting terminal of the wiring substrate is aligned with the wiring substrate and then adhered by adhesive agent.
When the content of the filler increases, the thermal expansion coefficient lowers. Thus, when a non-fluid resin layer of which the content of filler is large is formed between a semiconductor element and a wiring substrate, the stress due to the difference of the thermal expansion coefficients therebetween is effectively alleviated. Consequently, a semiconductor device with high connection reliability can be obtained.
In the case that a thermoplastic resin sheet containing high density of a filler is used, when bumps are thermally bonded, the thermoplastic resin layer swells out along the side peripheral surface of the semiconductor element. Thus, an underfill is formed in a good shape. In addition, the shape of the bumps can be controlled.
The non-fluid resin layer may be formed in the following method. In this method, photo-setting fluid resin is coated on a wiring substrate. The coated layer is exposed and developed with a mask having a predetermined pattern. Thus, a hardened resin layer having a pattern except for the connecting terminal can be formed.
According to the second aspect of the present invention, at the step of which bumps are heated, melted, and connected, with a support layer of a non-fluid resin layer formed on the connecting terminal formed surface of the wiring substrate, the height of the bumps (namely, the size of the space between the semiconductor element and the wiring substrate) is controlled. Thus, without a height controlling means for preventing the semiconductor element from lowering, the height of the bumps can be kept.
In addition, since a layer formed on a connecting terminal formed surface of a wiring substrate is a non-fluid resin layer, the resin does not adhere, reside, or interpose at the connection interface of the bumps. Thus, a connection defect does not take place. The thermal stress due to the difference of the thermal expansion coefficients between the semiconductor element and the wiring substrate is effectively alleviated with the bumps. Thus, a semiconductor device with high connection reliability can be accomplished.
In addition, when the thickness of the non-fluid resin layer is adjusted, the height of the bumps can be controlled. Thus, high and thin bumps (the height of the bumps is larger than the diameter thereof) can be accomplished. Consequently, the pitch of bumps can be narrowed corresponding to the pitch of the connecting pad.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawings.